Helmet-Mounted Displays: - USAARL - The - U.S. Army

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B. Joseph McEntire
impacting surface and the ability of the outer shell to distribute the load.
The Army aviation community has eliminated the hemispherical impact
anvil from the performance requirement and selected the flat anvil only.
This decision was based on the fact that in the Army helicopter crash
environment, hemispherical impact surfaces were rarely struck while flat
surfaces were prominent. The hemispherical impact anvil presents a point
loading threat to the helmet. To defeat this threat, the helmet shell must be
rigid enough to resist local deformation and distribute the impact load and
the energy liner must possess either an increase thickness or an increase
crush resistance load. Designing a helmet system to defeat the point
loading threat has typically resulted in increased mass.
The impact attenuation material used in Army aviation helmets has
been predominantly expanded bead polystyrene. This material and molding
process is well known and inexpensive. It also possesses desirable impact
attenuation characteristics, such as good energy attenuation and low
rebound . This material is also predominant in the motorcycle and bicycle
protective helmets. Other impact attenuation materials are available and
should be considered. Polyurethane is one such material. Polyurethane has
been and continues to be used by military aviators in the United Kingdom.
Frangibility
Frangibility of helmet components is required when the total head
supported mass creates an unacceptable risk of neck injury. The purpose
of incorporating frangible (automatically detachable) devices into the
helmet assembly is to remove the mass from the helmet, thus reducing the
risk of neck injury. The AN/PVS-5 NVG, when used by Army aircrew,
were attached to the SPH-4 helmet with “hook and pile” fasteners and
elastic tubing. This method did not allow the goggles to easily or
consistently detach during a crash. During ANVIS development, the
attachment mechanism was designed with a spring loaded “ball and socket”
engagement which allowed the NVG to separate from the mount when
exposed to an 10 to 15 G loading. This mechanism has performed well in
the Army helicopter crash environment. The IHADSS HDU, which is a
monocular CRT display, is also easily attached and detached from the
helmet mount. Mounted on the right lower edge of the helmet shell, the
HDU also detaches from the helmet during crash loadings, and actual crash
experience has shown it to perform well.
Both, the ANVIS and IHADSS HDU detachment mechanisms operate
when the device is exposed to crash loads and its dynamic inertia loads